No surrender, say the fighting Klitchkos

Ukrainians have nowhere to retreat and will carry on defending their homeland from Russian invaders, Kyiv mayor and former heavyweight boxing champion Vitali Klitschko say.

Flanked by his heavyweight boxing star brother Wladimir, Klitschko spoke via Skype from Kyiv as Russia's invasion of Ukraine entered its second week.

Moscow was shelling several Ukrainian cities as its main assault force remained at a halt north of the capital.

"Thousands, already tens of thousands are killed in this war against Ukraine. And this number will unfortunately only grow," said Klitschko, speaking from a shelter he is working from in Kyiv.

"We are not going to surrender. We have nowhere to retreat."

He said about a half of the city's three million people had left. Authorities had also evacuated many orphans to Poland and Germany, he added.

"The situation is tense, people are worried," he said, adding projectiles were constantly "hitting residential blocks or important infrastructure sites."

"The city needs peace and peaceful skies... Kyiv residents have been in their basements non-stop for a week now because of air raid alarm going off all the time."

Klitschko said Russia was trying to surround Kyiv as well and called on Russians to challenge President Vladimir Putin.

"You are an instrument at the hands of one man with one ambition - to resurrect the Soviet Union... For ambitions of one man, we are paying a very high price."

Putin said his "special military operation" was meant to protect Russians persecuted in Ukraine, a former Soviet republic that now wants to join NATO and the European Union.

The boxing brothers, whose mother is ethnically Russian, said Ukraine was always home to Ukrainians, Russians, Jews and other religions and